In recent years, large-scale photovoltaic (solar cells and the like) or solar thermal (photovoltaic collecting systems and the like) power generation systems with power outputs of as much as several hundred thousand kilowatts have been widespread. For example, as the solar thermal power generation systems, which generate electric power by collecting the photovoltaic, a beam-down type solar thermal power generation system has been known (refer to Patent Document 1). In the beam-down type solar thermal power generation system, the photovoltaic is reflected toward an upper side of a center portion of the system by heliostats, which are reflecting mirrors, and the reflected light is collected to a cavity receiver (heat receiving unit) set at a lower side by a large-sized reflecting mirror called as a center reflector.
FIG. 3 shows a schematic plan view of a beam-down type solar thermal power generation plant 5. Multiple heliostats 30, which are reflecting mirrors, follow the sun, and collect their reflected light to a center reflector 35, which is a second reflecting mirror. The reflected light thus collected to the center reflector 35 is then concentrated onto, and thereby heats, a heat medium, such as a molten salt, which is circulating in a heat-receiving unit. A power generation system in which the thermal energy of the heat medium thus heated is converted to an electric energy by a steam turbine or the like is an example of the aforementioned solar thermal power generation system.
FIG. 4 is a side view of the heliostats 30. Each of the heliostats 30 is constituted by combining multiple facets 31, which are small reflecting mirrors. In FIG. 4, three heliostats 30 are mounted on a common rotating mechanism 32, and are connected to one another with a tilting mechanism 33. The heliostats 30 are thus configured to automatically track the sun in accordance with a signal from a photovoltaic tracking sensor 34. Each heliostat 30 is configured as follows for the purpose of enhancing the light collection efficiency. As in a cross-sectional view shown in FIG. 5, the angles of the facets 31 can be adjusted by facet adjusting bolts 37 so that the facets 31 should form a pseudo curved surface 36, which is a rotating conical surface. FIG. 6 is a top view of the heliostats 30, and the multiple facets 31 form a reflecting mirror.
A problem has been pointed out that heliostats, which are reflecting mirrors used in the system, or panels of a photovoltaic power generation system get dirty with dust, bird droppings, or the like, so that the power generation efficiency is decreased. However, no countermeasure against the problem has been established yet. At present, a method in which cleaning work is manually performed has been employed. However, this method involves a hard work, and sometimes requires the setting of a scaffold because the work may have to be performed in a high place. For these reasons, this method leads to a decrease in work efficiency and an increase in costs. In addition, the poor workability of this method possibly causes a risk of damaging a glass surface of a reflecting mirror, a panel, and the like. If the power generation systems will be increased in scale further in the future, the problem will tend to become large functionally and economically. The photovoltaic and solar thermal power generation systems are often constructed in desert areas, such as those in the Middle East. For this reason, panels and reflecting mirrors are likely to get dirty with sand dust, and also, the manual work may be difficult because of a high temperature in some case.
In short, these manual cleaning works have the problem of the safety of a worker and the problem of the risk of damaging a glass surface of a panel or a mirror, which is a target of the work. In addition, there is also a problem in which an increase in the scale of a power generation system leads to an increase in cleaning costs in proportion, so that the scale effect cannot be achieved. For the photovoltaic or solar thermal power generation system itself, a larger scale effect can be achieved as the system increases in size, or the power generation costs can be suppressed by the mass production effect. Consequently, as a power generation system is increased in scale, the proportion of maintenance costs required for the cleaning work for the system is relatively increased.
On the other hand, it can be considered that the cleaning work is performed by using a robot. The applicant of the present application filed an application relating to a robot that cleans a window of a building or a glass of an aquarium in the past, and considered to apply a cleaning system remotely operated using a rotary brush extensively to the power generation system (refer to Patent Document 2).
However, when the frequency and absolute amount of the cleaning work are increased along with an increase in the scale of the power generation system, the initial costs for installing a robot are increased because of an increase in the size of the robot and the like. Moreover, the operating costs are increased because of energy consumption of the robot and the remote operation manually conducted, in turn leading to deterioration of the economy. For these reasons, it seems highly likely at the current circumstances that the application of the above-described cleaning system is difficult.
Specifically, if the unit size of solar cell panels or reflecting mirrors is increased along with an increase in the scale of the power generation system (a solar cell panel having a size of 6 m×12 m has already existed), a robot of a single unit type for cleaning these panels or mirrors is increased in size, resulting in a large-sized, very heavy machine (robot) such as a gantry crane, for example. The price (initial costs) of the robot also becomes high. The cleaning work of a panel or a reflecting mirror itself is merely cleaning a glass surface, which does not require the aforementioned robot to have a large output. However, the motive power (operating costs) for moving the robot itself while supporting the weight of the robot is increased to an extent that cannot be ignored. This is inefficient and deteriorates the economy. Further, the remote operation requires manpower, so only the work environment for the worker is improved but the economy is not improved.
Furthermore, robots have to be designed and manufactured separately in conformity with the specifications, such as the size of panels and reflecting mirrors, of individual photovoltaic power generation and solar thermal power generation systems. This is also a cause of an increase in the price of the robots.